Numerical Simulation Analysis On Two Cases Of Rainstorms Under The Effect Of Low-Level Jets On The East Side Of The Helan Mountain

The rainstorm process on the east side of the Helan Mountain is always accompanied with low-level jet.Firstly,synoptic pattern and mesoscale characteristics analyses of two rainstorm cases which occurred on the east side of the Helan Mountain were carried out using intensive observation data,radiosonde data and radar observation data,one case was occurred from July 18 to 19,2018(hereafter the "7.18" process,which is a linear convective system rainstorm)under the influence of the southerly low-level jet and the other case from June 4 to 5,2017(referred to as the "6.04" process,which is a nonlinear convective system rainstorm)under the influence of the southeast low level jet.And then numerical sensitivity experiments were conducted to study the impact mechanism of two types LLJ on the formation and development of rainstorms.In view of the deviation of WRF model in the simulation of the heavy precipitation center on the south side of the Helan Mountain during the "6.04" rainstorm,cyclic assimilating C-band radar reflectivity data experiment with WRF model and its three-dimensional variational system WRFDA was carried out,and the effect of assimilation of C-band radar reflectivity data on the simulation of the heavy precipitation period of this rainstorm was discussed.The main conclusions are as follows:(1)Synoptic analyses of the two rainstorm processes indicate that the "7.18" rainstorm was a local short-time rainstorm with concentrated rain bands which was the results of the interaction of upper trough as well as the southerly low-level jet.On the other hand,the "6.04" process was a large-scale continuous precipitation process formed under the influence of the large stable trough and the southeast low level jet.The water vapor in two cases came from the South China Sea and the Bay of Bengal respectively.Mesoscale convective system in the case "7.18",was a linear cloud belt,while for "6· 04" rainstorm was a larger scale convective cloud system.(2)The numerical simulation results of the two rainstorm processes show that the distribution characteristics of the precipitation simulated by the model are basically consistent with those of the observation station data,except that the location of the rain belt is closer to the mountain side,and the dynamic convergence area on the left side of LLJ southwards before the rainstorm occurred in the process of "7.18" is located in the west of Ningxia region.The main function of low-level southerly jet is to provide favorable water vapor conditions for the precipitation process.When the southerly jet is strengthened,the uplift movement in front of the Helan Mountain is enhanced,which is conducive to the formation of heavy precipitation near the mountain and the release of latent heat of condensation.During the process of "6.04",the southeast low-level jet moved relatively slowly,and the northern region of Ningxia was always on the left side of LLJ.At this time,LLJ not only provided favorable water vapor conditions for the formation of rainstorm,but also provided favorable dynamic mechanism for the formation of large-scale sustained heavy precipitation with dynamic and radiation cooperation on its left side.(3)The results of numerical sensitivity tests show that the precipitation reduction area of the "7.18" process during the weakened LLJ is located near the Helan Mountains,while the precipitation reduction area of the "6.04" process occurs in the plain area of Ningxia,and the dispersion field and pseudo-equivalent temperature characteristics of the two rainstorm processes after the weakened LLJ change significantly.The influence of LLJ on the "7.18" process is mainly reflected in the water vapor transport effect and the dynamic effect of strengthening the vertical motion of the windward slope,while the influence on the "6.04" process is mainly reflected in the dynamic convergence lifting effect on the left side of the LLJ.(4)The FLEXPART model was used to analyze the backward track of water vapor transport characteristics in the two rainstorm cases.The results showed that the particle tracks were concentrated during the "7.18" rainstorm,and the middle water vapor transport above 1600 m from the surface made an important contribution to the precipitation on the east side of the Helan Mountain.During the "6.04" rainstorm,the southeast water vapor transport channel was the main water vapor source,and its height was mostly below 1600 m.The simulation results after LLJ weakening showed that the water vapor transport at low level in both processes was weakened,but the water vapor loss during the water vapor transport in the "7.18" process was significantly greater than that in the "6.04" process.(5)The numerical test results of "6.04" rainstorm assimilation C-band radar showed that the results of 12 h cyclic assimilation test(RAD12)significantly improved the scores of 10 mm,25 mm and 50 mm threshold precipitation.The radar reflectance factor diagnosed after assimilation is in good agreement with the radar observation.The vertical profile of radar echo can well simulate the process of cold and warm air confluence before and after the precipitation extreme time and the weather front rising to form precipitation.This indicates that choosing an appropriate assimilation window to fuse C-band radar observation data into the initial field of the model can significantly improve the simulation effect of short-term heavy precipitation.